Large-scale power generators often use gas turbine engines to supply power to a geographic region. Reliable start up of these engines is critical to ensure that power outages do not occur.
Electric starters are known for starting small engines, but the special requirements of large engines have generally made them inappropriate for use with large engines. Many gas turbines in the 20 to 50 MW range are derivatives of aircraft engines (known as “aeroderivatives”) and, as such, they initially retained the lightweight, high-performance pneumatic starters from the initial design. As applications evolved, the pneumatic starters were replaced with hydraulic starters because weight is less of a concern in ground-based gas turbine engines. The next logical step is to apply electric motors to replace the hydraulic starter units.
One of the challenges in designing electric starters is to create a high-performance electric motor with minimal packaging size and total weight. From the currently available electric motor technology (i.e., induction, switched reluctance, permanent magnet), permanent magnet motors offer the highest performance density.
More particularly, electric starters rely on electric motors to convert electrical input power into mechanical power in the form of rotational torque, which in turn is used to start an engine. High-density motors produce a significant amount of heat and therefore require an effective heat dissipation mechanism to avoid overheating of the motor components. The electrical resistance of the stator winding in the motor and the fluctuating magnetic field passing through the metallic stator and rotor are both sources of much of the heat in the motor. Moreover, the size and high power density of the motor generate too much heat to be effectively dissipated through air cooling. Liquid coolant may be used to dissipate heat, but coolant flow is typically provided by a positive displacement pump that is driven by the engine, which is in turn rotated by the starter, making the coolant unavailable during motor startup. Also, the motor torque and its associated current will be highest at startup, further aggravating the lack of heat dissipation from the motor.
Electric motors used in close proximity to gas turbine engines must be capable of operation in an environment that may contain flammable gases. Under certain conditions, electric motors may develop hot spots and/or electrical shorts, which can increase the risks of operating electric motors in a hazardous environment.
Additionally, currently-known electric motors require periodic replacement or relubrication of various components, such as rotor bearings. Although pre-lubricated bearings are often used, these bearings have limited useful life because the oil/grease tends to leak, evaporate, and break down over time. The resulting reduction in lubricity causes wear particles from the bearing to form and accumulate in the lubricant, causing the lubricant to act as an abrasive and hasten component wear.
There is a desire for an electric starter having a motor that can be effectively cooled and isolated to make it usable in a variety of applications and environments.